Using an emittance technique with a fast CO2 laser heating of glass samples, the high-temperature absorption spectra in the near-infrared region of ultrapure and colored (Co-, Cu-, Mn-, and Ni-doped) glasses are measured. The effects of higher glass temperatures on these absorption spectra are explained in the framework of the ligand field theory. Thus, the temperature-dependent absorption bands of the previous transition metal ions are assigned to electronic transitions among the ligand field energy levels of these ions. In particular, spectral shifts, spectral broadening, and changes in absorption strength are ascribed to changes in the structural symmetry of the ionic sites in the glass matrix and to changes of the ligand field strength at increasing temperatures. Besides, the temperature-dependent Rosseland mean absorptions of the sulfate fined soda lime silicate glass melts, colored with the previous transition metal ions, are derived from the absorption spectra. Combining all the data, semiempirical correlations are derived, which predict the Rosseland thermal radiation properties as a function of glass temperature and of glass redox chemistry. The latter property involves the temperature-dependent concentration of the specific valency of the coloring ions, determined independently, e.g. by a Gibbs minimization redox calculation tool.

Two innovative experimental techniques for measuring the high temperature near infrared optical spectra of glass melts are compared. The critical experimental features of both techniques, one based on transmission and the other based on emittance measurements, are reviewed. Typical results of both techniques, including high temperature spectra and values for the Rosseland mean absorption coefficient and thermal radiation conductivity versus temperature for similar glass melts, are compared. The study is focused on sulfate fined soda lime silicate glass melts colored with iron oxide and chromium oxide and on the effect of the glass redox state on the thermal radiation conductivity. It is shown that essentially different measuring principles provide consistent results for similar glass melt types, that is, colors. Using the high temperature spectra of a large variety of (colored) glasses, a new semi-empirical model is developed for predicting the Rosseland radiation conductivity of arbitrary sulfate fined soda lime silicate glass melts, colored with iron oxide and chromium oxide. By separating the effects of (a) the temperature-dependent redox state, (b) the high temperature changes in ligand field strengths and (c) the glass matrix, the model reliably predicts the Rosseland radiation conductivity, with a chemical analysis of the glass as input only.

Glass melting is often viewed as a black box process: raw materials and energy are used as input, and the glass product results. The complex processes which occur in the glass melting tank are of vital importance for the resulting amount and quality of the glass obtained and the conditions should be such that the occurrence of unwanted side effects such as refractory corrosion and high emission of waste gases are minimized. Improving the glass making process requires complex modeling and experimental observation of the detailed sub processes. CelSian Glass & Solar has developed and uses several unique dedicated laboratory set ups and methods to simulate in detail the major process steps which take place in industrial glass melting furnaces. The facilities are tailor-made for studying industrial glass melting processes, simulating industrial conditions, furnace atmospheres, batch compositions, use of industrial refractories and heating curves. In this paper, two of the laboratory set-ups are described in detail and examples are given which demonstrate their application.Testing the flue gas attack of regenerator refractory materials to compare the corrosion resistance of the refractories.Use of high temperature melting observation and evolved gas analysis to study foaming and ways to combat this.

Several chemical reactions take place during the heating of a glass forming raw material batch. These reactions are accompanied by a change in batch volume, the release of several (batch) gases and occurrence of melting phases, gas bubbles and foam. Evolution of gas species, such as CO, CO2, O-2, and SO2 can be monitored as a function of temperature. From this information, the reaction mechanisms and temperatures can be derived during the fusion of the batch into a molten glass.CelSian developed and uses experimental equipment to enable the observation of the melting-in of a batch in transparent vitreous crucibles and simultaneous analysis of the evolved gases from the batch during heating up, melting and fining. The furnace atmosphere during melting and fining is controlled and the effect of different batch compositions on the temperature of fining-onset and fining gas production can be measured. For instance, changes in the furnace atmosphere can strongly influence the fining temperatures and fining efficiency.The equipment is applied to study the effect of:Batch pretreatment, such as grinding of the batch or pelletizing;Addition of cokes or different sulfate/ coke ratios in the batch;Exchange one raw material for another type;Changing oxygen or water vapor level in the furnace atmosphere;Batch humidification;Addition of oxidizing agent;on the melting-in behavior, foam formation and fining gas release. The experimental facility and method will be described in this paper and a few relevant examples for industrial glass production will be shown and discussed.This experimental method provides important information for glass technologists to optimize the raw material chemistry and glass melting conditions to improve melting and fining whilst limiting foam formation. This paper shows and discusses the application of this equipment to examine the effect of batch preparation techniques (coarse, fine batch, pellets) on melting-in and fining behavior.

At least 90 mass% of world's glass production uses some sulphate raw materials to improve melting kinetics and fining (removal of gas bubbles and dissolved gases) of glass melts. The batch reactions, especially in the presence of organic components or cokes, will influence the decomposition of sulphates or result partly in conversion to sulphides during batch melting. After batch melting, the remaining sulphur species, further react in, or with, the freshly molten glass, still containing sand particles and gaseous inclusions. Reactions may occur at different temperatures, associated with different glass melt viscosity levels. Evolved gas analysis of different batches, in specific (dry, wet, nitrogen) or simulated furnace atmospheres, is applied to investigate gas release (CO, CO2, SO2, O-2) and sulphur reactions during batch heating and glass melt fining. This information is essential to predict foaming and fining behaviour and final glass redox (colour of final glass) for industrially applied raw material batch compositions and to find methods for reducing SOx emissions. Sulphate in batch reacts with cokes or char originating from organic components. This often results in sulphide formation. After melting-in of most batch components, sulphide and sulphate react and form SO2 gas at temperature levels of about 1100-1350 degrees C. This release of SO2-gas, causes bubble growth at these temperatures with viscosity levels above 50 Pa s. Higher temperatures are required to ensure sufficiently rapid removal of these bubbles from the melt by buoyancy forces. Residual sulphate may decompose at higher temperatures, depending on the residual sulphate content after the disappearance of sulphide from the melt. Thermal decomposition of sulphate only takes place when using batches without high carbon or organic material contents. In that case, bubble growth and bubble ascension in common soda-lime-silica glass melts are effective above 1450 degrees C.